Use rainwater for your container pond. Tap water contains various additions dependent on where you are in the Country - including chlorine or chloramines which need to be reduced before the plants are added.
Or use Chlorine Guard which is an eco-friendly liquid treatment of the chlorine, chloramines and heavy metals that can be found in tap water that are harmful to plants, fish and wildlife.
Chlorine Guard should be used every time you top up your pond with tapwater throughout the season.
Include sufficient oxygenating plants in your planting scheme for the size of the container pond so the water will remain fresh and not turn stagnant.